This invention relates to composite materials. More specifically, this invention relates to composite friction materials which exhibit high, stable, coefficients of friction over a wide temperature range.
The elastomeric materials heretofore proposed for use as friction materials have generally proven to be unsatisfactory when exposed to high ambient working temperatures such as encountered, for example, in clutch and brake applications in heavy duty service vehicles. Typically, such materials have been based on heat-hardenable resins such as phenol-aldehyde resins which tend to heat-decompose under the high peak and bulk temperature conditions created by the sustained and/or heavy loading forces experienced in the clutch and brake systems of these vehicles while operating. As a result of this decomposition, the physical properties of these materials typically deteriorate, and the consequent softening of the material and dispersal of the products of heat decomposition generally interfere with the functioning of the friction unit. Furthermore, many times after friction material comprising a partially heat-decomposed heat-hardenable resin has cooled, the material will exhibit a less stable coefficient of friction than did the original material.
These conditions, as well as other problems associated with these and similar friction materials, result in a loss of efficiency in the friction unit and unreliability in the service vehicle, which is highly undesirable.
Many attempts have been made to obviate the problems associated with the polymers in general use as friction material basis. Many different resins have been experimented with, in attempts to obtain a friction material which possesses a high, stable coefficient of friction over a wide temperature range. Modification of the heat-hardenable resins with other polymeric materials has been attempted. Many of these friction material formulations have not performed well. Other formulations have required multi-step procedures which are costly in terms of labor and frequently in terms of the material used in these formulations.
Importantly, also, many of these known friction materials require a bonding agent to affix them to the backing plate or "core" portion of the friction element. This requirement severely restricts the scope of the molding methods and mold configurations employable in forming these friction elements. In injection molding, for example, the bonding agent is subject to scuffing during the molding process, which deactivates or destroys the bond and renders this molding process useless with these friction elements. In general, where bonding agents must be utilized, only compression molding and relatively simple mold configurations can be employed in the process of molding the friction element.
In order to obtain a friction material with a usefully high coefficient of friction which is stable over a wide temperature range, the industry has most usually used nonresilient inorganic friction materials such as sintered bronze. Although the friction characteristics of this and similar metallic materials have been generally satisfactory under high temperature conditions, the high modulus or lack of resiliency of these materials and their resultant inability during operation to conform to the friction element mating surface and absorb adequate energy result in relatively high wear rates and shortened life. Furthermore, great care must be taken in the type of oil used in conjunction with such friction materials during use to ensure that the desired coefficient of friction is not impaired.
It is therefore an object of this invention to provide a friction material composition with a high, stable coefficient of friction over a wide range of dynamic operating conditions.
It is a further object of this invention to provide a friction material composition with high dynamic and static coefficients of friction over a wide temperature range.
It is a further object of this invention to provide a friction material composition which can readily be bonded to a metal core material.
It is an additional object of this invention to provide a friction material composition which can be compression molded, and which can be molded in conjunction with complex mold configurations.
It is also an object of this invention to provide a conformable, long-wearing friction material composition with a high, stable coefficient of friction over a wide temperature range.
Other objects and purposes of this invention will be apparent to those skilled in the art from the disclosure contained herein.